Although the main functions of the digestive system were once believed to include little beyond the breakdown of food into usable energy for the body and elimination of waste, the digestive tract is also home to 100 trillion microorganisms, known as the gut flora. The majority of these microorganisms are bacteria, with a small percentage consisting of fungi and protozoa. The functions of the gut flora are complex enough to resemble those of an organ, leading some researchers to refer to the gut flora as a "forgotten organ".
Excellent run-down of gut flora, exploring its types, its role in immune and systemic health, and how it all works to keep us healthy (or in some cases, unhealthy!).